7k Cad was a 3.73 with the 305 TBI, and a 3.42 (or close) with the LT1 260hp. Other mods included what was basically the cop suspension, upgraded radiator, HD trans, and some other things. If I stick a diesel in there i'm planning on having the trans rebuilt or uprated for towing, and hunting down a bigger radiator. Swapping on 9C1 suspension parts or some other handling upgrade i've wanted to do ever since I got it.
Although a 6.2 diesel would have even less horsepower than the 305 i'm hoping a wider range of ultimate ratios in the transmission, or a range box, will let me both tow slow or run the highway just fine. Following the same strategy i'd bet dropping to a 4.10 or 4.56 would tow fine with the diesel those weights. The pickups were towing heavy weights with low rears afterall.
And to other yes i'm looking at boattail trailers. I'm hoping to aeromod everything.
Whats tricky is that it will be a flat bed, and my mods will have to be some kind of expanding frame with strong tarps/soft sided aero of some sort if that works. An enclosed trailer is too much resistance for usually dense low loads.